The study looked at how changes in German tax laws affected the progression of income tax. Before 2009, tax progression was strongest for middle-income earners and decreased for high earners, with the richest not showing much progression. After 2009, capital income was taxed separately at a lower rate. The researchers used tax data from 2001 to 2006 to see how this change impacted overall income tax progression. They found that the new tax system reduced progression for high-income earners, especially the very wealthy.